Accepted Manuscript Review Sulfated Tin Oxide (STO)-Structural Properties & Application in Catalysis: A Review Ravi Varala, Venugopalarao Narayana, Sripad R. Kulakarni, Mujeeb Khan, Abdulrahman Alwarthan, Syed F. Adil PII: DOI: Reference:
S1878-5352(16)30002-8 http://dx.doi.org/10.1016/j.arabjc.2016.02.015 ARABJC 1852
To appear in:
Arabian Journal of Chemistry
Received Date: Accepted Date:
9 December 2015 26 February 2016
Please cite this article as: R. Varala, V. Narayana, S.R. Kulakarni, M. Khan, A. Alwarthan, S.F. Adil, Sulfated Tin Oxide (STO)-Structural Properties & Application in Catalysis: A Review, Arabian Journal of Chemistry (2016), doi: http://dx.doi.org/10.1016/j.arabjc.2016.02.015
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Sulfated Tin Oxide (STO)-Structural Properties & Application in Catalysis: A Review
Ravi Varala,*,a,c Venugopalarao Narayana,b Sripad R. Kulakarni,c Mujeeb Khand, Abdulrahman Alwarthand, and Syed F. Adild,* a
Polymers department, Institute de Fisica de Sao Carlos, University of Sao Paulo-Sao Carlos Campus, P.O. Box: 369/13560-970, Brazil b Research Scientist, Neuland Pharma Research Lab, Bonthapalli, Hyderabad, Telangana, India c Department of Chemistry, RGUKT-Basar, Mudhole, Adilabad-504107, Telangana, India d Department of Chemistry, College of Science, King Saud University, P.O. 2455, Riyadh 11451, Kingdom of Saudi Arabia Tel.: +966114670439 *E-mail for correspondence:
[email protected] (S.F.A.),
[email protected] (R.V.) Abstract Catalysis is an important area of chemistry, with an extensive amount of work going on in this area of sciences, towards synthesis and evaluation of newer catalysts. There are many reports for different conversion reactions such as oxidation, reduction, coupling, alkylation, acylation for which various catalysts have been used such as mixed metal oxides, metal nanoparticles, metalorganic complexes and many others. Among the many catalysts reported, the one catalyst that caught our attention due to its exploitation for a plethora of organic conversions is the sulfated tin oxide (STO), which is due to the low cost, greater stability and high efficiency of the catalyst. In this review, we have attempted to compile data about the structural properties of STO, it’s applications as catalysts in various organic synthesis are presented. The literature data up to 2014 was collected and considered for the review. Keywords: Sulfated tin oxide; catalysis; reusability; heterogeneous reactions; biodiesel; organic transformations
Table of Contents 1. Introduction 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8
FT-IR spectroscopy Effects on the STO catalysts due to sulfate content on its surface Surface modification studies on tin oxide STO acidity: A solid-state NMR spectroscopy perspective Physicochemical characteristic of mixed metal oxide of sulfated Sn and rare earth metals XRD analysis TPD studies of the STO catalysts Scanning electron microscopic (SEM) analysis
1
2. Esterification and transesterification 2.1 2.2 2.3
Free fatty acids (FFA) esterification Waste cooking oil transesterification Esterification of free fatty acids (FFAs) in crude palm oil: Effects of iron precursor, calcination temperature and sulfate concentration 2.4 Optimized preparation of moringa oleifera methyl esters 2.5 Transestrification reaction using biodiesel as co-solvent 2.6 SnO2-SiO2: Croton megalocarpus oil for bio-diesel production 2.7 Biodiesel by catalytic reactive distillation powered by metal oxides 2.8 Esterification and transesterification on Fe2O3-doped sulfated tin oxide catalysts 2.9 Esterification and transesterification on Al2 O3-doped sulfated tin oxide solid acid catalysts 2.10 Synthesis of sulfated silica-doped tin oxides and their high activities in transesterification 2.11 Application of sulfated tin oxide in transesterification and bio-diesel production a. The heterogeneous transesterification of Jatropha curcas oil b. Esterification of free fatty acids in crude palm oil c. Synthesis of biodiesel via acid catalysis d. Biodiesel fuel production in fixed bed reactor under atmospheric pressure: e. STO-Catalyzed esterification and etherification reactions 3. Catalytic application in organic synthesis 3.1 Synthesis of 7-hydroxy-4-methyl coumarin using Pechmann condensation method 3.2 Baeyer-Villiger oxidation and acetalization of cyclic ketones 3.3 Synthesis of 2,4,5-triaryl-1H-imidazole derivatives 3.4 Dehydration of xylose 3.5 Friedel-Crafts acylation of toluene 3.6 Deprotection of silyl ether 3.7 Synthesis of 2,4-diphenyl-4,6,7,8-tetrahydrochromen-5-one 3.8 C3-alkylation and O-alkylation of 4-hydroxycoumarins 3.9 Efficient one-pot synthesis of 7,8-dihydro-2H-chromen-5-ones 3.10 Facile transesterification of ketoesters 3.11 Aldol condensation and benzoylation 3.12 Synthesis of β-aetamido ketones and aryl-14H- dibenzo (a.j)xanthenes 3.13 Selective dehydration of sorbitol 3.14 One-pot synthesis of naphthopyranopyrimidines 3.15 Synthesis of pyrimidopyrimidines 3.16 Catalytic pyrolysis of cellulose: Synthesis of light furan compounds 3.17 Mesostructured sulfated tin oxide and its high catalytic acitivity in esterfication and Friedel-Crafts acylation 2
3.18 3.19 3.20 3.21 3.22 3.23 3.24 3.25 3.26
Alkylation of phenol with methanol Selective nitration of phenol Alkylation of hydroquinone with tert-butanol Direct oxidation of n-heptane to ester over modified sulfated SnO2 catalysts under mild conditions Dealkylation of anisole to phenol using lanthanum-sulfated SnO2 Olefin polymerization and hydrogenation catalysts Synthesis and hydrolysis of dimethyl acetals Ethanol Oxidation Catalysis in direct coal liquefaction
4. Conclusion 5. Acknowledgement 6. References
1. Introduction Due to their excellent physico-chemical and fascinating surface properties, metal oxides represent the important class of materials which have profound implications in the field of catalysis(Ren, Ma et al. 2012). These materials have been widely applied as solid catalysts, either as active phases or as support materials(Gawande, Pandey et al. 2012). So far, various metal oxides have been extensively exploited as industrial catalysts for several commercially important reactions (Arena, Deiana et al. 2015). Metal oxide catalyst is either used in its pure form, or in many cases it is supported on other oxides, where the catalytic activity is enhanced by the interaction between two oxides(Park, Baker et al. 2015).Particularly, in the field of heterogeneous catalysis, metal oxides have been extensively applied for several important organic transformations, due to their excellent acid–base and redox properties(Célérier and Richard 2015). Among different metal oxides, tin oxide (SnO2) has attracted immense interest of the scientific community, due to its dual valency and its ability to attain more than one oxidation state(Batzill and Diebold 2005). Usually, SnO2 prefer to possess the oxidation states of 2+ or 4+, which greatly promotes the variation in the composition of surface oxygen, and thus, the SnO 2 exhibit varied surface properties, which has a significant importance in catalysis. Furthermore, due to the excellent acidic, basic, oxidizing and reducing surface properties, SnO 2 acts as an active heterogeneous catalyst for many important organic reactions.From the earlier observations, it has been proved that the efficiency of a heterogeneous catalyst depends on the 3
intrinsic catalytic activity of its component in addition to its texture and stability. The mineral form of SnO2 is called ‘cassiterite’ (Figure 1), and it is the main ore of tin(Greenwood and Earnshaw 1984). In tin chemistry, SnO2 is the most vital raw material. This is colourless solid that is diamagnetic and amphoteric in nature.
Figure 1 Tin oxide It has been reportedly used as catalyst in addition to the vanadium oxide for the synthesis of carboxylic acids and acid anhydrides by oxidation of aromatic compounds. It is being utilized in various ways as an opacifier and white colorant in ceramic glazes(Searle 2008), as a polishing powder(Jensen 2002) in glass coating and in gas sensing (Watson 1984). The correct choice of the additives is very prominent in fine tuning the surface properties of a catalyst. Attempts were made as a mean to improve the tin oxide catalyst by adsorbing various anions such as sulfate or phosphate anions onto the tin oxide. The increase in the surface acidity of the modified oxide leads to the raise in activity of the catalyst. A strong acidity generated with the modification of metal oxides with sulfate anion which is even stronger than 100% sulfuric acid and thus acting as super acid catalysts for several commercially important reactions. Acid catalysis has a vital industrial application, which is widely employed in the various isomerization, alkylation and cracking reactions that are used to upgrade oil and hence plays a crucial and important role in the petroleum industry. Large scale polymerization processes involve acids as catalysts and display activity for interconversion of ethanol and ethylene (hydration/dehydration reactions). Although with limited lifetimes, these sulfated metal oxides possess excellent ability to catalyze isomerization reactions, i.e. conversion of straight chain alkanes to more highly branched isomers, at low temperatures, which is a characteristic of very strong acid catalysts.
4
Arata and his group has exploited a range of active sulfated oxides including those based on SO42-/ZrO2, SO42-/SnO2, SO42-/Fe2O3, SO42-/SiO2, SO42-/TiO2, SO42-/HfO2 and SO42-/Al2O3(Arata 1990; Arata 1996; Arata 2009). It was reported that in order to obtain a catalyst with high activity, the sulfation must be performed on the amorphous precursors of the hydroxides or oxyhydroxides, however this was not found true for alumina (Al2O3). Among, various sulfation reagents available, either dilute sulfuric acid or an aqueous solution of ammonium sulfate has been commonly used by researchers. Sulfate, an additive among many has been detected to favor the formation of the tetragonal phase, while monoclinic form is one that is commonly found(Norman, Goulding et al. 1994). A number of diverse proposals have been made to explain the nature of the active catalytic site, mostly based across the symmetry of the coordinated sulfate ion(Brown and Hargreaves 1999).However, a number of symmetries for SO42- are possible in principle as depicted in Figure 2. They can be distinguished by examination of the sample S=O stretching region (1200 to 900 cm-1) in the infra-red spectra(Nakamoto 1986). Both C2v and C3v forms have been in general exploited in the literature.
Figure 2 Different SO42- symmetries (Brown And Hargreaves 1999). The sulfated tin(IV) oxide is one of the promising candidates for exhibiting the strongest acidity on the surface. The acid strength of sulfated tin oxide is proved to be higher than that of sulfated zirconia (Table 1, entries 1-2).
5
Acidity measurement: Table 1 Solid super acids and their acidic strength (Arata, Matsuhashi et al. 2003) Catalyst
Calcination temperature
Highest acid strengtha (Ho value)
SO4/SnO2
550 °C
−18.0
SO4/ZrO2
650 °C
−16.1
SO4/HfO2
700 °C
−16.0
SO4/TiO2
525 °C
−14.6
SO4/Al2O3
650 °C
−14. 6
SO4/Fe2O3
500 °C
−13.0
SO4/SiO2
400 °C
−12.2
WO3/ZrO2a
800 °C
−14.6
MoO3/ZrO2
800 °C
−13.3
WO3/SnO2
1000 °C
−13.3
WO3/TiO2
700 °C
−13.1
WO3/Fe2O3
700 °C
−12.5
B2O3/ZrO2
650 °C
−12.5
a
Acid strengths were estimated by color change of Hammett indicator, TPD of pyridine, TPR of furan, and catalytic activity for various reactions The literature is abundant with reports where in researchers have attempted characterization and employing the STO for various chemical transformations. A few characterization techniques are compiled below. 1.1 FT-IR spectroscopy: The IR spectra of sulfated tin(IV) oxide (xSTO) in comparison with pure tin(IV) oxide (TO) were shown in Figure 3(Clearfield, Serrette et al. 1994; Jogalekar, Jaiswal et al. 1998; Mekhemer, Khalaf et al. 2005; Mekhemer 2006). The appearance of the bands at 1382, 1190, 1155 and 1079 cm-1 confirmed the presence of sulfate group.
6
Figure 3 FTIR spectra for xSTO in comparison with TO catalysts(Khalaf, Mansour et al. 2011) The above observed features are in general attributed to triply bridging sulfate and bridged bidentate sulfate, as represented in the Figure 4. O O
S O M
O M
O S
O
O
O
M
M
Structure I
O M
O M
Structure II M = Sn
Figure 4 Triply bridging sulfate (I) and bridged bidentate sulfate (II)(Khalaf, Mansour et al. 2011)
7
Figure 5 FT-IR spectra of STO calcined at different temperatures(Dabbawala, Mishra et al. 2013) Dabbawala et al. prepared the STO and studied the effect taken palce due to calcination temperature using FT-IR. The spectra of the sulfated tin oxide catalysts is shown in the Figure 5. The bands/peaks at 1165, 1038, and 975 cm-1 in the spectrum clearly indicate the presence of sulfated group and a bidentate chelation mode between the sulfate and tin. By the existence of the sulfate groups on the catalyst surface, a strong acidic nature can be realized. The acidity of solids used as catalysts or catalyst carriers has been the subject of various investigations and reviews for a long time(Sheldon 2000; José da Silva and Lemos Cardoso 2013). It was learnt that by interaction with the active components (support effect) or by providing acidic sites which may make the acid catalyst bifunctional, thus influencing the properties of a catalyst. The acid sites on metal oxides are believed to be due to the surface hydroxyl groups or a charge imbalance localized on the surface. The deficient amounts of localized electrons result from a local imbalance between positive and negative charges of the constituents and act as a Lewis acid for catalytic reactions. By substituting the metal ion with other metal ions possessing different charges a charge imbalance can be expected and new acid sites could be generated on the surface of doped and or mixed metal oxides(Connell and Dumesic 1986; Hudlicky, Claeboe et al. 1999). 8
It is reported that some sulfated metal oxides produce super acid materials with surface acidity and much larger surface areas compared with those metal oxides without sulfate(Sheldon 1993). In general, the reactions catalyzed by strong acids, many of these super acid compounds are found to be very good catalysts, it is mainly because of the features like acidity and surface area. With enlarged acid surface properties that come from the sulfation process as well as the existence of two metals that are able to absorb H 2 and henceforth, the new system can be considered as a bifunctional catalyst. Sulfation enhances the strength of the weakest Lewis acid sites but poisons the strongest. In the case of highly loaded samples it also creates Bronsted acidity(Waqif, Bachelier et al. 1992). 1.2. Effects on the STO catalysts due to sulfate content on its surface Khder and coworkers assessed the effect of calcination temperature and sulfate content on the acidity, structure and catalytic activity of sulfated tin oxides. Numerous catalysts were synthesized with different sulfate content using different thermal treatment temperatures as shown in the Table 2. An increase in SO4−2 ions load resulted in consequent increase in SnO2 surface area irrespective of the calcinations temperature. It was observed that decrease in the surface area of both catalysts SO42-/SnO2 as well as SnO2 resulted in increase in the calcination temperature. Based on these observations, the authors suggested that the presence of sulfate ions on the surface of tin oxide prevents a higher reduction in surface area provoked by an increase in the calcinations temperature, while Table 3 demonstrates the notable effect observed by both increase in the calcination temperature and increase in the sulfate content (Khder, El-Sharkawy et al. 2008). Table 2 BET surface area measurements obtained from SO42-/SnO2 catalysts synthesized at temperature of 400, 550, 750 ºC (Khder, El-Sharkawy et al. 2008) Surface area (m2/g)
SO4-2 content on SnO2 catalyst (wt.%) 0 1 5 15 20 30
400 ºC 61.2 69.9 76.2 90.2 101.3 129.5
550 ºC 46.3 52.3 64.7 80.8 87.3 100.8
750 ºC 20.4 28.5 36.5 57.2 62.5 70.1
9
Table 3 Acidic properties of the SO42-/SnO2 catalysts synthesized at temperatures of 400, 550, 750 ºC (Khder, El-Sharkawy et al. 2008) Calcination temperature (ºC) SO42-
content on
400
NAS
550
Ei -1
NAS
750
Ei -1
NAS
Ei -1
Pyridine loss (wt %)
t (ºC)
SnO2 catalyst (wt%)
(meq g )
(mV)
(meq g )
(mV)
(meq g )
(mV)
0
0.20
−84
0.35
+27
0.14
−127
0.210
424.5
1
0.31
−70
0.51
+194
0.24
−109
0.448
460.8
5
0.38
−61
0.66
+316
0.30
−97
0.667
468.4
15
0.50
−27
0.83
+479
0.42
−54
1.430
479.4
20
0.56
+9
0.97
+510
0.51
−41
1.841
482.0
30
0.68
+25
1.30
+555
0.60
−23
2.440
482.5
NAS: acid sites number; Ei: initial electrode potential indicates the maximum acid strength of the sites; t: pyridine loss temperature Kdher and coworkers observed that when the catalyst contained the highest NAS and the highest amount of sulfate ions, the highest conversions occurred in the acetic acid esterification with amyl alcohol and reusability studies of the catalyst were also studied. The graphical representation of conversion results reported are given in Figure 6 (A), while Figure 6(B) gives the results obtained from the reusability study of the catalyst. Based on this study, Kdher and coworkers proposed that the presence of a covalent S=O bond may lead to the generation of strong Lewis acidity of the sulfate species, which act as electron withdrawing species, which is followed by the inductive effect, thereby making the Lewis acid strength of Sn+4 stronger as demonstrated by arrows in the Figure 7. On addition, the Lewis acid sites are converted to Bronsted acid sites in the presence of water, via proton transfer(Khder, El-Sharkawy et al. 2008).
10
100
A
1.8 1.6
70
1.4
60
1.2
50
1
40
0.8
30
0.6
20
0.4
total no. of acid sites (mEq/g)
80
B
95
conversion of amyl alcohol (%)
90
conversion of amyl alcohol (%)
100
2
90 85 80 75
-2
20SO4 /SnO2 -2
30SO4 /SnO2
70 65
10 0
5
10
15
20
25
30
0.2
60 Fresh
Run 1
Run 2
Run 3
Run 4
sulfate (wt%)
Figure 6 (A) Relation between sulfate content, esterification reaction of acetic acid with amyl alcohol at 250 °C and surface activity. (B) Effect of reusability of catalysts at 250 °C amyl alcohol/acetic acid molar ration 2:1(Khder, El-Sharkawy et al. 2008)
Figure 7 A surface structure of sulfated tin oxide(Khder, El-Sharkawy et al. 2008) 1.3. Thermal stability studies of tin oxide In order to study the surface properties, active sites and its strength, preparation of the sulfated tin oxide catalyst, has been carried out under different conditions and along with addition of other metal oxides by various research groups.
11
Figure 8 TGA and DTA profiles for (a) tin(IV) gel (TH), and (b) sulfated tin(IV) oxide (6STO), wloss is the mass fraction(Khalaf, Mansour et al. 2011) Khalaf reportedly studied the thermal stability of sulfated tin oxide (STO) and compared it with the tin gel. It was reported that the TG profile was found to be different with STO displaying three step mass loss while the precursor, unsulfated tin gel (SnO2.xH2O) undergoes thermal degradation in two step mass loss. The precursor tin gel (SnO2.xH2O) and STO shows a mass loss of ~19.9%, when heated up to 1273 K. The thermogram obtained is given in Figure 8(Khalaf, Mansour et al. 2011). 1.4. STO Acidity: A solid-state NMR spectroscopy perspective Yu et al. studied the acidic properties of SnO2, SO42-/SnO2, ZrO2 and SO42-/ZrO2, by using solid-state NMR spectroscopy (Figures 9 (A) and (B)). It was shown that new acidic sites were created when SO42- was able to interact with the hydroxyl groups of SnO2 and ZrO2. 1
H,
13
C, and
31
P MAS NMR spectra of the above mentioned molecules showed that only on
SO42-/SnO2 a Bronsted acid site was present. The acid strength of SO42-/ZrO2 was weaker than that of SO42-/SnO2(Yu, Fang et al. 2009). Table 4 shows characterized and the textual properties of the prepared samples. Table 4 Textural properties of various samples (Yu, Fang et al. 2009)
112
Average crystal size/nm 15.6
Sulfate content/wt% -
White
SO42-/ZrO2
134
12.2
2.30
White
SnO2
28
16.0
-
Yellow
SO42-/SnO2
93
4.8
3.31
Yellow
Samples
SBET(m2 g-1)
ZrO2
Color
12
The authors compared the 1H NMR spectra obtained from the sulfated metal oxides and the corresponding metal oxides. It was found that the peaks with the larger chemical shifts were observed in case of sulfated metal oxides. It clearly proves that when the metal oxides were treated with sulfuric acid, the new acid sites were formed.
A
B
Figure 9 (A) 1H MAS NMR spectra of ZrO2 (a), SO42-/ZrO2 (b), SnO2 (c) and SO42-/SnO2 (d). (B) 1H MAS NMR spectra of pyridine-d5 loaded on ZrO2 (a), SO42-/ZrO2 (b), SnO2 (c) and SO42-/SnO2 (d) (Yu, Fang et al. 2009) 1.5. Physicochemical characteristic of mixed metal oxide of sulfated Sn and rare earth metals Binary mixed oxides of tin with three rare earth elements viz. La, Ce and Sm was reported by Jyothi et al. The co-precipitation method and sulfate treatment was employed, which was executed by treating the mixed hydroxides with sulfuric acid or ammonium sulfate(Jyothi, Sreekumar et al. 2000). The strength as well as the distribution of acid sites depend on both the 13
preparation method as well as on the mixed metal oxide composition. In the oxidative dehydrogenation of cyclohexanol and cyclohexane, the rare earth modified sulfated tin oxide catalysts proved to be more active when compared to the corresponding mixed oxide systems and sulfated tin oxide. The cerium promoted catalysts showed excellent selectivity towards dehydrogenation products, among the various sulfated oxide systems studied. 1.6 XRD analysis XRD analysis is a famous technique for the analysis of crytalline nature of the metal salts and various phase changes that take place in the mertal salt upon change in physilogial conditions. Numerous tin(IV) oxide catalysts, pure and surface-doped with different loading levels of sulfate, have been synthesized and characterized by Khalaf et al. using different characterization techniques(Khalaf, Mansour et al. 2011). It is revealed from the structural investigation of the catalysts that the tin(IV) gel has the formula SnO 2·2H2O. The addition of sulfate will not affect the crystalline structure of tin(IV) oxide (tetragonal phase) but it decreases the crystallite size and as a result increases the specific surface area.
Figure 10X-ray powder diffractograms for xSTO (x=different loding leves) in comparison with TO catalyst (Khalaf, Mansour et al. 2011) The sulfated samples, with different loading levels of sulfate by weight (6STO, 12STO and 18STO), XRD diffractograms obtained at calcination temperature 873 K are shown in Figure 10. It is evident that all patterns are characteristic of pure SnO2 phase with tetragonal rutile structure at 2 =26.54, 33.82 and 51.74(Khalaf, Mansour et al. 2011). 14
Similar studies were carried out by Dabbawala and coworkers, wherein they prepared the STO and studied the phase transitions that take place due to calcination temperature using XRD, the obtained XRD spectrum is given in Figure 11. It is reported that the decomposition of SnSO4 to SO42-/SnO2 took place at a calcination temperature of 350 °C. For all the samples prepared, above the calcination temperature of 350 °C, a disappearence of characteristic diffraction peaks of SnSO4 indicates the complete conversion to SO42-/SnO2.
Figure 11 XRD patterns of STO calcined at different temperatures (O-SnO2) (Dabbawala, Mishra et al. 2013) Du et al. prepared a mesostructured sulfated tin oxide catalyst (MST) possessing large surface area (172 m2/g) while employing block copolymer as template, followed by the sulfation of the sample. They employed X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TG), and nitrogen adsorption methods to characterize the MST catalysts. The XRD analysis yielded spectrum with peaks at 2θ 27, 34, 38, 52, 65 as given in the Figure 12, which revealed that the samples possess tetragonal phase of crystalline SnO2(Du, Liu et al. 2006).
15
Figure 12 Small-angle XRD patterns of MST-500 (a) and MST-550 (b); and wide-angle XRD patterns of MS-500 (c) and MST-550 (d) (Du, Liu et al. 2006) 1.7. TPD studies of the STO catalysts Furuta and coworkers measured the sulfated oxides acidity employing pyridine adsorption and ammonia adsorption TPD(temperature-programmed desorption ) studies. The two pyridine adsorption characteristics bands measured via FT-IR (i.e., 1445 and 1541 cm−1, resp.) confirms the presence of Lewis and Bronsted acids sites. It was observed that the band correspondent to Lewis acid sites had higher intensity in the compounds with a higher amount of SO42- ions, and it is due to electron withdrawing effect of sulfate group, which thus increases the Lewis acidity of the Sn cations (Figure 13).
16
Figure 13 Electron withdrawing effect of sulfate group on sulfated tin oxide By means of TPD technique, Figure 14 shows the nitrogen content that remained on the catalyst surface, which was determined by elementary analysis of nitrogen. It clearly indicates that over a wide range of temperature, the tin catalysts possesses a much higher acidity(Furuta, Matsuhashi et al. 2004).
Figure 14 Ammonia-TPD profiles of STO-1, STO-2 , and SZ (sulfated zirconia) catalysts (Furuta, Matsuhashi et al. 2004) 17
1.8. Scanning Electron Microscopic (SEM) Analysis Dabbawala and coworkers subjected the prepared STO samples to SEM-EDX studies to understand the morphology and elemental analysis. The presence of sulfur, tin and oxygen was observed from the SEM-EDX spectrum as seen in Figure 15. It was also observed that the STO has a cubic morphology of the size 30-35 nm(Dabbawala, Mishra et al. 2013).
Figure 15 SEM images and SEM-EDX spectrum of STO-450(Dabbawala, Mishra et al. 2013) 2. Esterification and transesterification: Esterification reaction is an important reaction industrially as it is employed for the production of important chemicals; such as polyesters, biodiesel and glycerol, which is 18
extensively used in pharmaceutical formulations and food industries, hence research towards finding a right catalyst has been extensively studied and sulfated tin oxide catalyst has been reported for various such reactions mentioned below: 2.1. Free fatty acids (FFA) Esterification Moreno et al. synthesized sulfated tin oxides with different sulfate contents from the hydroxylated tin oxide using precipitation method. They proved that upon sulfation, the SnO 2 crystal growth could be inhibited. The SO42- species strongly binds to the SnO2 surface and thus stabilizing its crystalline size against sintering(Moreno, Jaimes et al. 2011). The esterification of oleic acid increased with the increase in the surface acidity and a maximum conversion was achieved at 0.3 wt% sulfate content and 773 K calcination temperature. The reaction of SnCl2⋅2H2O with NH4OH resulted in the precipitation of Sn(OH) 2⋅H2O which on filtration, and the solid impregnation with H2SO4, followed by thermal treatment leads to the synthesis of SO4-2/SnO2 solid catalyst. It was observed from the thermogravimetric data that the surface area of the catalyst was reduced at higher temperatures. Moreno and group formulated an important factor of the relationship between the solid catalyst activity and the acidity using Hammett Indicators. This relation can be easily understood in the esterification reaction of oleic acid with ethyl alcohol as illustrated in Figure 16. The catalyst with the highest NAS was more active; however, it does not have the highest surface area; this fact further supports that in the acid solidcatalyzed esterification reactions, the acid sites are also essential. The BET surface area data and thermogravimetric analysis of the prepared catalyst was concluded in Table 5. Table 5 Textural properties of the SnO2 and SO42-/SnO2 catalysts(Moreno, Jaimes et al. 2011) Entry 1
2
3 4
Catalyst-sulfate (wt%) Temperature (K) SnO2 (0) 773 SO42-/SnO2 (0.45) 773 SO42-/SnO2 (0.45) 873 SO42-/SnO2 (0.30)
Specific surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (Å)
(TGA) mmol SO3/g catalyst
62
0.051
70
-
99
0.054
25
0.201
88
0.052
28
0.137
87
0.050
28
0.194 19
773
6
773 SO42-/SnO2 (0.45) 673
85
0.060
28
0.129
92
0.060
26
0.194
2.6
50
oleic acid conversion (%)
2.4 40
2.2 2
30
1.8 20
1.6
10
1.2
1.4
total no. of acid sites (mmol/g)
5
SO42-/SnO2 (0.15)
1 0
0.8 1
2
3
4
5
6
A
Figure 16 Catalytic activity and total number of acid sites of the catalysts: (1) SnO2-773K, (2) SO4-2/SnO2-0.45-673K, (3) SO4-2/SnO2-0.45-773K, (4) SO4-2/SnO2-0.45-873K, (5) SO4-2/ SnO20.3-773K and (6) SO4-2/SnO2-0.15-773K during the esterification reaction of oleic acid with ethanol (molar ratio 1:10) at 353K, after 4 h of reaction(Moreno, Jaimes et al. 2011) 2.2. Waste cooking oil transesterification Lam and co-workers synthesized a superacid sulfated tin oxide catalyst by employing the impregnation method(倉員貴昭 2011). To increase the catalytic activity of SnO2, it was mixed with SiO2 and Al2O3 respectively at different weight ratio and the bimetallic effect was probed.
20
100 90 80 70 Yield, %
60 50 SnO2 SnO2 - SiO2 (1) SnO2 - SiO2 (3) SnO2 - SiO2 (5) SnO2 - Al2O3 (1) SnO2 - Al2O3 (3) SnO2 - Al2O3 (5)
40 30 20 10 0 90
120
150
180
210
0
Reaction Temperature, C
Figure 17 Effect of reaction temperature on FAME (fatty acid methyl ester) yield using SO4-2/SnO2 and its sulfated mixed metal oxide. Reaction condition: methanol:oil ratio 10, catalyst loading 3wt%, reaction time 3 h. All samples were calcined at 300 0C for 2 h(倉員貴昭 2011) The transesterification reaction temperature (100-200 oC) on the yield of FAME using different bimetallic catalysts was studied at different weight ratio (Figure 17). It was observed that increasing the temperature from 100 to 150 oC, authors noted tremendous increase in the yield of FAME for all types of catalyst tested. 2.3. Esterification of free fatty acids (FFAs) in crude palm oil Nuithitikul et al. studied the activities of sulfated iron-tin mixed oxide catalysts, which were prepared using co-precipitation method, for esterification of free fatty acids in crude palm oil(Nuithitikul, Prasitturattanachai et al. 2011). The varied parameters for the synthesis were iron precursor, sulfate concentration and calcination temperature. Iron sulfate was found to be the best precursor from the observations made. With the increase in sulfate content, the activity of the catalyst was found to increase. In reuse of the catalyst, its stability was improved by the addition of the iron oxide to the sulfated tin oxide. The optimum calcination temperature was 450 °C. Hence, sulfated iron-tin oxide was found to be the best catalyst for the esterification of fatty acids in comparison with STO. The research group mainly concentrated on the content of iron and tin based catalyst in the esterification reaction. The effect of different sources of iron as starting materials as [FeCl3, Fe(NO3)3, and Fe2(SO4)3] in the esterification reaction was also investigated. The catalyst, 30% wt.SO4-2/Fe2O3-SnO2, with the highest surface area, strongest acid sites and highest NAS, was 21
the most active. The results obtained are attributed to the interaction of sulfate ion with Sn(II) which favors the formation of two acid sites as described in the Figure 18(Nuithitikul, Prasitturattanachai et al. 2011).
Figure 18 Lewis and Bronsted acid sites(Nuithitikul, Prasitturattanachai et al. 2011) One interesting observation reported is that the effect was more pronounceable when the iron precursor was iron nitrate and it is due to nitrate ions which can also act as an electron withdrawing group, thereby increasing the Lewis acid strength of tin catalysts. The Figure 19 represents the comparison study of 79 wt.% SO4-2/Fe2O3-SnO2 catalysts and 79 wt.% SO4-2/SnO2 catalysts activity and their stability. The authors made another observation that the sulfate tin oxide catalysts containing iron (III) oxide were having highest catalytic activity in the FFA esterification reaction and also found to be more stable. They attributed the increase in the catalytic activity of the sulfated tin oxide catalysts due to the presence of iron cations, which increased the Lewis acidity of the catalyst.
22
Figure 19 Reusability of 79 wt% SO42-/Fe2O3-SnO2 (III) calcined at 450 oC. Reaction time = 3h(Nuithitikul, Prasitturattanachai et al. 2011) 2.4. Optimized preparation of Moringa oleifera methyl esters For the preparation of biodiesel, Moringa oleifera fatty acids were found to be potent starting material(Kafuku and Mbarawa 2010).The SiO2 enhanced sulfated tin oxide (SO42-/SnO2-SiO2) super acid solid catalyst is used for the production of biodiesel from Moringa oleifera oil. 2.5. Transestrification reaction using biodiesel as co-solvent For the transesterification reaction catalyzed by SO42-/SnO2-SiO2, a solid acid catalyst, Lam and his research group studied the use of biodiesel as co-solvent(Lam and Lee 2010). They obtained a high FAME yield of 88.2% (almost 30% more than in the absence of co-solvent) when THF is used as a co-solvent in shorter reaction time (1.5 h). The research group optimized the reaction conditions as: 150 °C reaction temperature, 6 wt% of catalyst loading, methanol to oil ratio as 1:15. 2.6. SnO2-SiO2: Croton megalocarpus oil for bio-diesel production Kafuku and his group investigated the conversion of non-edible oil, from Croton megalocarpus oil, to the methyl esters (biodiesel). The catalyst used was sulfated tin oxide enhanced with SiO2(Kafuku, Lam et al. 2010). Although, the Croton megalocarpus oil was found to contain high free acid content, a yield up to 95% was obtained from the oil without any 23
pre-treatment step under the optimized reaction conditions. The optimized reaction conditions were: temperature of 180 °C, 2 h and 15:1 methanol to oil molar ratio, 3 wt% constant catalyst concentration and 350-360 rpm stirring speed. 2.7. Biodiesel by catalytic reactive distillation powered by metal oxides Kiss et al. outlined the issues associated with biodiesel´s present production processes as well as the properties and the use as a renewable fuel. The research group proposed a novel feasible esterification process which depends on the catalytic reactive distillation. They illustrated a metal oxide super acid solid catalyst based synthesis of the biodiesel via fatty acid esterification(Kiss, Dimian et al. 2007).Among the metal catalysts prepared niobic acid, sulfated zirconia, sulfated titania, and sulfated tin oxide were found to be the best candidates. 2.8. Esterification and transesterification on Fe2O3-doped sulfated tin oxide catalysts A variety of Fe2O3 (0.2-3%) doped sulfated tin oxide solid catalysts were prepared by employing the co-precipitation process followed by sulfation and calcination methods. Zhai et al. observed that by addition of small amounts of Fe2O3 to the STO, the number of active/strong acid sites were increased and hence the Fe2O3 doped STO catalysts were found to be more active, for the esterification of lauric acid with methanol and transesterification of triacetin with methanol, than the undoped ones(Zhai, Nie et al. 2011). The STO catalyst containing 1% of Fe2O3 exhibits highest activity for the biodiesel production as illustrated in the Figure 20.
(A)
(B)
Figure 20 (A) Esterification activity of STO (sulfated tin oxide) and STF (sulfated tin oxide– Fe(OH)3) series catalysts as a function of reaction time. (B) Transesterification activity of ST and STF series catalysts as a function of reaction time (Zhai, Nie et al. 2011)
24
2.9. Esterification and transesterification on Al2O3-doped sulfated tin oxide solid acid catalysts Zhai research group prepared a series of Al2O3 (0.5-3%) doped sulfated tin oxide catalysts by using the co-precipitation process. They investigated the catalytic activity of these catalysts in the esterification reaction of lauric acid with methanol and transesterification of triacetin with methanol.(ZHAI, YUE et al. 2010) The results showed that the catalytic activity improved markedly by the addition of Al2O3 to STO. The remarkable activity of the catalyst was due to the increase in the number of acid sites. The sulfated tin oxide catalyst doped with 1 mol% of Al2O3 found to exhibit highest activity. The lauric acid esterification was 92.7% and triacetin transesterification was 91.1%, with the help of this catalyst, after 6 h and 8 h respectively. 2.10. Synthesis of sulfated silica-doped tin oxides and their high activities in transesterification Xiao et al. synthesized a series of sulfated tin oxide catalysts doped with silica with large surface areas (113-188 m2/g) by employing hydrothermal synthesis, sulfation and calcination methods. These catalysts were characterized by XRD, FT-IR, TEM, nitrogen isotherms, and TG techniques. The authors observed that these catalysts are composed of tetragonal nanocrystalline tin oxides and amorphous silica. These silica doped sulfated tin oxides catalysts found to show excellent catalytic activity for the transesterification of triacetin with methanol when compared with the conventional sulfated tin oxide catalysts(Du, Liu et al. 2008). 2.11. Application of sulfated tin oxide in transesterification and bio-diesel production The doping of sulfated tin oxide with aluminum resulted in better catalytic activity and acidity than the zirconia and titanium doping on sulfated tin oxide as reported by Guo et al(Guo, Yan et al. 2008). The incorporation of aluminum on SO42-/SnO2-Al2O3 via co-condensation method at Sn/Al ratio of 9:1 and calcined at 500 °C, gave the best activity. From the FT-IR results, it was observed that the active sites are due to Sn, which was chelated with sulfuric acid. By introducing aluminum, the number of the sulfate groups attached on the surface increased. And the observations made from thermogravimetric analysis, further strengthened the catalyst’s activity. Lam et al. observed a yield of 92.3% with SO42-/SnO2-SiO2 with a weight ratio of 3:1 at 150 °C, 3 wt% of catalyst, and 15:1 methanol to oil molar ratio in 3 h(Lam, Lee et al. 2009). Calcination temperature was pivotal for these catalytic reactions (300 oC). BET surface area and crystal size of sulfated tin oxide catalysts were also investigated along with the activation energy 25
of the NH3 desorption super acid sulfated tin oxide catalyst as shown in Table 6.
The acid
acidic strength of catalyst can be measured from the activation energy of the NH 3 desorption. Table 6 Activation energy for NH3 desorption, BET surface, and crystal size for super acid sulfated tin oxide catalysts Activation
Correlation
BET (surface
Crystal size
energy (kJ/mol)
factor (R2)
area)
(nm)
SnO2
5.69
0.9540
8.32
60.7
2-
SO4 /SnO2, 300 ºC
7.25
0.9687
6.77
42.9
SO42-/SnO2, 400 ºC
7.18
0.9715
-
59.6
SO42-/SnO2, 500 ºC
6.01
0.9877
-
62.0
SO42-/SnO2-SiO2, 300 ºC
9.40
0.9991
13.90
47.7
SO42-/SnO2-Al2O3, 300 ºC
7.51
0.9925
14.04
45.4
Catalyst activation
2.11.a The heterogeneous transesterification of Jatropha curcas oil Jatropha curcas seeds contain non-edible oil yielding excellent results in the biodiesel industry. It consists of a high free fatty acid content that can be converted into methyl esters by transesterification using a basic catalyst. The sulfated tin oxide enhanced with SiO2 (SO42−/SnO2SiO2), a super acid solid catalyst, was used by Kafuku et al to prepare methyl esters from the Jatropha curcas oil(Kafuku, Lee et al. 2010). They set up the experiments using the response surface central composite design (CCD). The reaction parameters considered were the reaction temperature, reaction period and methanol to oil ratio. CCD was used to investigate linear, quadratic and cross-product effects of the given reaction parameters on the yield of jatropha methyl esters. The value of α was fixed at 2. A maximum yield (97%) of methyl ester was obtained at a temperature of 180 °C, 1:15 molar ratio of methanol and oil for a reaction period of 2 hours. 2.11.b Esterification of free fatty acids in crude palm oil The production of biodiesel from the crude palm oil using the homogeneous base catalyst contains large amounts of free fatty acid (FFA) and it also results in the soap formation and thereby reducing the yield of biodiesel. To overcome these problems, the free fatty acids need to be esterified to their esters by using an acid catalyst prior to alkaline catalyzed transesterification. A number of aluminum doped sulfated tin oxide catalysts were synthesized by using different 26
aluminum precursors/starting materials(Prasitturattanachai and Nuithitikul 2013). From the results, it was observed that the different aluminum precursors gave different activities of catalysts. The esterification reaction of free fatty acids with methanol using these catalysts found to follow the first-order kinetics. 2.11.c Synthesis of Biodiesel via Acid Catalysis The sulafted tin oxide catalysts, prepared from m-stannic acid, were found to exhibit higher catalytical activity than the sulfated zirconia catalysts for the esterification reaction of n-octanoic acid with methanol at temperature
